Phase plugs are usually disposed, in use, adjacent a diaphragm, the diaphragm being driven axially to generated sound waves; these sound waves are channelled by the phase plug so as to enhance the acoustic performance of the diaphragm. Conventional phase-plugs have an axisymmetric surface which closely follows the geometry of the associated diaphragm. The enclosed volume of air between the diaphragm and phase-plug must be sufficiently small to avoid loss of high frequency output due to acoustic compliance. To achieve maximum low frequency output the diaphragm must move with the largest possible displacement.
Some attempts have been made to shape the diaphragm in the axial direction so as to increase its stiffness, and thus improve its acoustic performance; however, the introduction of such shapes inevitably either reduces the clearance between the phase plug and the diaphragm (which is undesirable, as it risks the diaphragm impinging on the phase plug during operation, which has a drastic adverse effect on the sound quality), or it increases the volume of the cavity between the diaphragm and phase-plug, which is also undesirable. As a result of this, and other practical constraints, the size of the axial shaping is severely restricted in the part of the phase plug facing the diaphragm.